U.S. patent application number 11/226272 was filed with the patent office on 2007-03-15 for start-up apparatus for power converters.
Invention is credited to Wei-Hsuan Huang, Ta-Yung Yang.
Application Number | 20070058398 11/226272 |
Document ID | / |
Family ID | 37854889 |
Filed Date | 2007-03-15 |
United States Patent
Application |
20070058398 |
Kind Code |
A1 |
Yang; Ta-Yung ; et
al. |
March 15, 2007 |
Start-up apparatus for power converters
Abstract
A start-up apparatus for a power supply is presented. A charging
path from an input voltage to a holding capacitor is cut off after
the power converter starts up. The start-up apparatus includes a
transistor having a drain supplied with the input voltage, and a
source connected to the holding capacitor and an input of a
start-up control unit. An output of the start-up control unit
controls a switch and the transistor. The holding capacitor starts
to be charged as the transistor is turned on. Once a voltage across
the holding capacitor exceeds a start-up voltage, an internal
control circuit is powered via the switch. Meanwhile, the
transistor is turned off and the charging path is cut off.
Furthermore, the start-up apparatus provides a hysteresis threshold
voltage range for controlling the power converter.
Inventors: |
Yang; Ta-Yung; (Milpitas,
CA) ; Huang; Wei-Hsuan; (Taoyuan City, TW) |
Correspondence
Address: |
ROSENBERG, KLEIN & LEE
3458 ELLICOTT CENTER DRIVE-SUITE 101
ELLICOTT CITY
MD
21043
US
|
Family ID: |
37854889 |
Appl. No.: |
11/226272 |
Filed: |
September 15, 2005 |
Current U.S.
Class: |
363/16 |
Current CPC
Class: |
H02M 1/0006 20210501;
H02M 3/33507 20130101; Y10S 323/901 20130101; H02M 1/36
20130101 |
Class at
Publication: |
363/016 |
International
Class: |
H02M 3/335 20060101
H02M003/335 |
Claims
1. A start-up apparatus for starting up a power converter, which is
supplied with an input voltage and is connected to a holding
capacitor, comprising: a transistor, having a source, a gate and a
drain, said drain of said transistor is supplied with said input
voltage, said source of said transistor being connected to said
holding capacitor; a start-up control unit, having an input and an
output, said input of said start-up control unit being connected to
said holding capacitor and said source of said transistor, said
output of said start-up control unit being connected to said gate
of said transistor; and a switch, connected to said output of said
start-up control unit and said holding capacitor, wherein said
input voltage drives said transistor on and charges said holding
capacitor, wherein said start-up control unit draws a start-up
voltage from said holding capacitor to drive on said switch and to
provide a voltage across said holding capacitor for a control
circuit, wherein said start-up control unit also turns off said
transistor to cut off a charging path from said input voltage to
said holding capacitor.
2. The start-up apparatus as claimed in claim 1, said transistor is
a junction field effect transistor.
3. The start-up apparatus as claimed in claim 2, said start-up
apparatus further comprising a start-up resistor, said start-up
resistor having a first terminal supplied with said input voltage
and a second terminal connected to said drain of said
transistor.
4. The start-up apparatus as claimed in claim 2, said start-up
control unit comprising: a first p-transistor, having a source
connected to said holding capacitor and said source of said
transistor, said first p-transistor further having a gate connected
to said holding capacitor via a first resistor; a first
n-transistor, having a drain connected to said gate of said first
p-transistor, said first n-transistor further having a source
connected to a ground reference; a first clamper, connected between
a drain and said source of said first p-transistor; a second
clamper, connected between said drain of said first p-transistor
and a gate of said first n-transistor, wherein said gate of said
first n-transistor is connected to said ground reference via a
second resistor; and a buffer circuit, connected to said drain of
said first n-transistor, said holding capacitor, a control terminal
of said switch, and said gate of said transistor, said buffer
circuit operating in response to a switching of said first
n-transistor.
5. The start-up apparatus as claimed in claim 4, wherein said first
clamper and said second clamper break down successively in response
to a voltage increment across said holding capacitor, wherein said
first clamper and said second clamper respectively generate a first
clamping voltage and a second clamping voltage so as to provide a
hysteresis threshold voltage range.
6. The start-up apparatus as claimed in claim 4, said buffer
circuit comprising: a second p-transistor having a gate connected
to said gate of said first p-transistor, said second p-transistor
further having a source connected to said holding capacitor, said
source of said transistor and a first terminal of said switch; a
first inverter, having an input connected to a drain of said second
p-transistor, said first inverter further having an output
connected to said control terminal of said switch and said gate of
said transistor; and a third resistor, connected between said drain
of said second p-transistor and said ground reference.
7. The start-up apparatus as claimed in claim 1, said transistor is
a metal oxide semiconductor transistor.
8. The start-up apparatus as claimed in claim 7, said start-up
apparatus further comprising a start-up resistor, said start-up
resistor having a first terminal supplied with said input voltage,
said start-up resistor further having a second terminal connected
to said drain of said transistor;
9. The start-up apparatus as claimed in claim 7, said start-up
control unit comprising: a first p-transistor, having a source
connected to said holding capacitor and said source of said
transistor, said first p-transistor further comprising a gate
connected to said holding capacitor via a first resistor; a first
n-transistor, having a drain connected to said gate of said first
p-transistor, said first n-transistor further comprising a source
connected to a ground reference; a first clamper, connected between
a drain and said source of said first p-transistor; a second
clamper, connected between said drain of said first p-transistor
and a gate of said first n-transistor, wherein said gate of said
first n-transistor is connected to said ground reference via a
second resistor; and a buffer circuit, connected to said drain of
said first n-transistor, said holding capacitor, a control terminal
of said switch, and said gate of said transistor, said buffer
circuit operating in response to a switching of said first
n-transistor.
10. The start-up apparatus as claimed in claim 9, wherein said
first clamper and said second clamper break down successively in
response to a voltage increment across said holding capacitor,
wherein said first clamper and said second clamper respectively
generate a first clamping voltage and a second clamping voltage to
provide a hysteresis threshold voltage range.
11. The start-up apparatus as claimed in claim 9, said buffer
circuit comprising: a second p-transistor, having a gate connected
to said gate of said first p-transistor, said second p-transistor
further having a source connected to said holding capacitor, said
source of said transistor and said first terminal of said switch; a
first inverter, having an input connected to a drain of said second
p-transistor, said first inverter further having an output
connected to said control terminal of said switch; a third
resistor, connected between said drain of said second p-transistor
and said ground reference; a second inverter, having an input
connected to said output of said first inverter; and a second
n-transistor, having a source connected to said ground reference, a
drain connected to said gate of said transistor, and a gate
connected to an output of said second inverter.
12. The start-up apparatus as claimed in claim 9, said buffer
circuit comprising: a second p-transistor, having a gate connected
to said gate of said first p-transistor, said second p-transistor
further having a source connected to said holding capacitor, said
source of said transistor and said first terminal of said switch; a
first inverter, having an input connected to a drain of said second
p-transistor, said first inverter further having an output
connected to said control terminal of said switch; a third
resistor, connected between said drain of said second p-transistor
and said ground reference; a second inverter, having an input
connected to said output of said first inverter; a second
n-transistor, having a source connected to said holding capacitor
via a voltage divider, a drain connected to said gate of said
transistor, and a gate connected to an output of said second
inverter; and a charge pump capacitor, having a positive terminal
connected to said gate of said transistor and said drain of said
second n-transistor, said charge pump capacitor further having a
negative terminal connected to said output of said first inverter
and said input of said second inverter.
13. The start-up apparatus as claimed in claim 12, said charge pump
capacitor being utilized to accelerate turning on said
transistor.
14. A start-up apparatus for starting up a power converter, which
is supplied with an input voltage via a start-up resistor and is
connected to a holding capacitor and an auxiliary winding,
comprising: a start-up control unit, having an input and an output,
said input of said start-up control unit being connected to said
holding capacitor; and a switch, connected to said holding
capacitor and said output of said start-up control unit, said input
voltage charging said holding capacitor via said start-up resistor,
wherein said start-up control unit draws a start-up voltage from
said holding capacitor to drive said switch on, therefore a voltage
across said holding capacitor powers a control circuit via said
switch.
15. The start-up apparatus as claimed in claim 14, said start-up
unit comprising: a first p-transistor, having a source connected to
said holding capacitor and a first terminal of said switch, said
first p-transistor further having a gate connected to said holding
capacitor via a first resistor; an n-transistor, having a drain
connected to said gate of said first p-transistor, said
n-transistor further having a source connected to a ground
reference; a first clamper, connected to said source and a drain of
said first p-transistor; a second clamper, having a first terminal
connected to said drain of said first p-transistor, said second
clamper further having a second terminal connected to a gate of
said n-transistor, said gate of said n-transistor being connected
to said ground reference via a second resistor; and a buffer
circuit, connected to said drain of said n-transistor, said holding
capacitor, and a control terminal of said switch, said buffer
circuit operating in response to a switching of said
n-transistor.
16. The start-up apparatus as claimed in claim 15, said first
clamper and said second clamper break down successively in response
to the voltage increment across said holding capacitor, wherein
said first clamper and said second clamper respectively generate a
first clamping voltage and a second clamping voltage so as to
provide a hysteresis threshold voltage range.
17. The start-up apparatus as claimed in claim 15, said buffer
circuit comprising: a second p-transistor, having a gate connected
to said gate of said first p-transistor, said second p-transistor
further having a source connected to said holding capacitor and
said first terminal of said switch; an inverter, having an input
connected to a drain of said second p-transistor, said inverter
further having an output connected to said control terminal of said
switch; and a third resistor, connected between said drain of said
second p-transistor and said ground reference.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a start-up apparatus for a
power converter. More particularly, the present invention relates
to a start-up apparatus having a hysteresis threshold voltage range
to control the power converter.
[0003] 2. Description of Related Art
[0004] FIG. 1 shows a power converter comprising a switching
controller U.sub.1. The switching controller U.sub.1 is utilized to
control the power converter for regulating a stable output.
However, a supply voltage is needed for powering an internal
control circuit. Further referring to FIG. 1, as an input voltage
V.sub.IN is applied to the power converter, the input voltage
V.sub.IN will charge up a holding capacitor C.sub.ST via a start-up
resistor R.sub.ST. The switching controller U.sub.1 will then start
to switch a transistor T.sub.1 via a power switch Q.sub.1. After
the power supply is started up, an auxiliary winding N.sub.A of the
transformer T.sub.1 further powers the switching controller U.sub.1
via a diode D.sub.A.
[0005] The start-up resistor R.sub.ST works only for starting up
the power converter. After the power converter is started up, the
start-up resistor R.sub.ST only consumes unnecessary power and
becomes redundant. A power consumption P.sub.R of the start-up
resistor R.sub.ST can be expressed by equation (1). P R = ( V IN -
V DD ) 2 R ST ( 1 ) ##EQU1##
[0006] Power-related organizations in the world have set up a
series of specification for green-mode power saving. Therefore, it
is more desirable for power converter designers to make the
start-up resistor R.sub.ST open circuit or to provide a solution
without the start-up resistor for saving power, especially under no
conditions.
SUMMARY OF THE INVENTION
[0007] The present invention provides a start-up apparatus for a
power converter. A start-up resistor connected between an input
voltage and a switching controller is not essential. By cutting off
the charging path from the input voltage to a holding capacitor
after the power converter starts up, the power consumption can be
reduced. Further, the start-up apparatus of the present invention
provides a hysteresis threshold voltage range for turning on/off
the power converter.
[0008] The start-up apparatus comprises a transistor having a drain
supplied with the input voltage. A source of the transistor is
connected to the holding capacitor and an input of a start-up
control unit. An output of the start-up control unit is connected
to a gate of the transistor and a control terminal of a switch. The
switch further has a first terminal connected to the holding
capacitor and a second terminal connected to an internal control
circuit. Therefore, as the input voltage drives the transistor on,
the holding capacitor is charged up. When a voltage across the
holding capacitor exceeds a start-up voltage, the start-up control
unit will drive the switch on so as to provide the voltage of the
holding capacitor to the internal control circuit. Simultaneously,
the transistor is turned off so as to cut off the charging path
from the input voltage to the holding capacitor.
[0009] The start-up apparatus further comprises a first clamper and
a second clamper. The first and second clampers are implemented by
zener diodes. The first clamper having a first breakdown voltage
and the second clamper having a second breakdown voltage break down
successively in response to a voltage increment across the holding
capacitor. This provides a first clamping voltage and a second
clamping voltage to be the hysteresis threshold voltage range for
turning on/off the power converter.
[0010] The present invention also provides a start-up apparatus for
a power converter. The start-up apparatus is connected to a holding
capacitor and is connected to an auxiliary winding of a transformer
via a diode. The start-up apparatus is further coupled to an input
voltage via a start-up resistor. A start-up control unit of the
start-up apparatus has an input connected to the holding capacitor
and an output connected to a control terminal of a switch. The
switch has a first terminal connected to the holding capacitor and
a second terminal connected to an internal control circuit.
Therefore, the input voltage charges the holding capacitor via the
start-up resistor. As the voltage across the holding capacitor
exceeds a start-up voltage, the start-up control unit will turn on
the switch. Accordingly, the holding capacitor can power the
internal control circuit via the switch.
[0011] As described the same above, the start-up apparatus also
comprises a first clamper and a second clamper. The first clamper
and the second clamper break down successively in response to the
voltage increment across the holding capacitor. This provides a
first clamping voltage and a second clamping voltage to be the
hysteresis threshold voltage range for turning on/off the power
converter.
[0012] It is to be understood that both the foregoing general
descriptions and the following detailed descriptions are exemplary,
and are intended to provide further explanation of the invention as
claimed. Still further objects and advantages will become apparent
from a consideration of the ensuing description and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] These and other objects, features and advantages of the
present invention will become apparent to those skilled in the art
upon consideration of the following description of the preferred
embodiments of the present invention taken in conjunction with the
accompanying drawings.
[0014] FIG. 1 shows a circuit diagram of a traditional power
converter.
[0015] FIG. 2 shows a circuit diagram of a power converter
according to the present invention.
[0016] FIG. 3 shows the power converter having a start-up apparatus
according to a first embodiment of the present invention.
[0017] FIG. 4 shows the power converter having the start-up
apparatus according to a second embodiment of the present
invention.
[0018] FIG. 5 shows an equivalent circuit diagram illustrating a
metal oxide semiconductor transistor having parasitic
capacitors.
[0019] FIG. 6 shows the power converter having the start-up
apparatus according to a third embodiment of the present
invention.
[0020] FIG. 7 shows the power converter having the start-up
apparatus according to a fourth embodiment of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0021] FIG. 2 shows a circuit diagram of a power converter
according to the present invention. As an input voltage V.sub.IN is
applied to the power converter, the input voltage V.sub.IN will
start to charge a holding capacitor C.sub.ST via the switching
controller U.sub.2. As a voltage across the holding capacitor
C.sub.ST reaches a start-up voltage, the start-up apparatus powers
an internal control circuit 90 to switch a transformer T.sub.1 on
or off via a power switch Q.sub.1. Further, a start-up resistor
R.sub.ST can be inserted between the input voltage V.sub.IN and the
holding capacitor C.sub.ST so as to extend a start-up time of the
power converter.
[0022] As the switching controller U.sub.2 operates, an auxiliary
winding N.sub.A of the transformer T.sub.1 will charge the holding
capacitor C.sub.ST via a diode D.sub.A so as to power the switching
controller U.sub.2. Further, the start-up apparatus will cut off a
charging path from the input voltage V.sub.IN to the holding
capacitor C.sub.ST after the power converter is started up.
Accordingly, the power consumption of the power converter can be
reduced. Moreover, the start-up apparatus provides a hysteresis
threshold voltage range for the power converter to turn on/off the
power converter.
[0023] FIG. 3 shows a circuit diagram of the power converter having
the start-up apparatus according to a first embodiment of the
present invention. The switching controller U.sub.2 provides a
supply voltage V.sub.CC for powering an internal control circuit
90. According to the present invention, the start-up apparatus is
directly connected to the input voltage V.sub.IN or connected to
the input voltage V.sub.IN via a start-up resistor R.sub.ST. The
start-up apparatus is further connected to a holding capacitor
C.sub.ST. The start-up apparatus comprises a transistor 40, a
start-up control unit 80 and a switch 63. The transistor 40 is a
junction field effect transistor (JFET) having a negative threshold
voltage. The transistor 40 is turned off when a gate-to-source
voltage is lower than the negative threshold voltage.
[0024] Further referring to FIG. 3, the transistor 40 has a source,
a gate, and a drain. The drain of the transistor 40 is supplied
with the input voltage V.sub.IN. The source and the gate of the
transistor 40 connect to the start-up control unit 80. The start-up
control unit 80 comprises a first P-transistor 61 having a source
connected to the holding capacitor C.sub.ST, a gate connected to a
buffer circuit 82 and to the holding capacitor C.sub.ST via a first
resistor R.sub.1; a first N-transistor 65 having a drain connected
to the gate of the first P-transistor 61, a source connected to a
ground reference; a first clamper Z.sub.1 having a first terminal
connected to the source of the first P-transistor 61 and a second
terminal connected to a drain of the first P-transistor 61; and a
second clamper Z.sub.2 having a first terminal connected to the
drain of the first P-transistor 61 and a second terminal connected
to a gate of the first N-transistor 65. The second terminal of the
second clamper Z.sub.2 is further connected to the ground reference
via a second resistor R.sub.2.
[0025] The buffer circuit 82 comprises a second P-transistor 62
having a gate connected to the gate of the first P-transistor 61, a
source connected to the holding capacitor C.sub.ST; a first
inverter 71 having an input connected to a drain of the second
P-transistor 62 and an output connected to gates of the transistor
40 and the switch 63; and a third resistor R.sub.3 connected
between the input of the first inverter 71 and the ground
reference.
[0026] As described above, the first clamper Z.sub.1 and the second
clamper Z.sub.2 are zener diodes having a breakdown voltage
V.sub.Z1 and a breakdown voltage V.sub.Z2, respectively.
[0027] The first clamper Z.sub.1 and the second clamper Z.sub.2
break down successively in response to a voltage increment across
the holding capacitor C.sub.ST. This provides a first clamping
voltage V.sub.1 and a second clamping voltage V.sub.2 for the
hysteresis threshold voltage range of the start-up apparatus of the
present invention. The first clamping voltage V.sub.1 is also the
start-up voltage of the switching controller U.sub.2. The first
clamping voltage V.sub.1 and the second clamping voltage V.sub.2
are respectively expressed by following equations (2) and (3):
V.sub.1=V.sub.Z1+V.sub.Z2+V.sub.R (2) V.sub.2=V.sub.Z2+VR (3) Where
V.sub.R is a voltage across the second resistor R.sub.2.
[0028] Referring to FIG. 3, a control terminal of the switch 63 is
connected to the output of the buffer circuit 82. A first terminal
of the switch 63 is connected to the holding capacitor C.sub.ST. A
second terminal of the switch 63 is connected to the internal
control circuit 90. Once the input voltage V.sub.IN is applied to
the power supply, the transistor 40 of the start-up apparatus is
instantly turned on. Therefore, the input voltage V.sub.IN starts
to charge up the holding capacitor C.sub.ST. As the voltage across
the holding capacitor C.sub.ST reaches the first clamping voltage
V.sub.1, the first clamper Z.sub.1 and the second clamper Z.sub.2
will break down accordingly. The voltage V.sub.R will be built
across the second resistor R.sub.2. The voltage V.sub.R therefore
turns on the first N-transistor 65. This pulls the gate of the
first P-transistor 61, the output of the buffer circuit 82, the
control terminal of the switch 63, and the gate of the transistor
40 to the ground reference. Therefore, the first P-transistor 61
and the switch 63 are turned on accordingly.
[0029] The transistor 40 is turned off since its gate is connected
to the ground reference. A charging path from the input voltage
V.sub.IN to the holding capacitor C.sub.ST is cut off. In such a
manner, the power consumption of the power converter can be
reduced. The first clamper Z.sub.1 is short-circuit while the first
P-transistor 61 is turned on. As the switch 63 is turned on, the
energy stored in the holding capacitor C.sub.ST, which is provided
from the auxiliary winding N.sub.A of the transformer T.sub.1,
powers the internal control circuit 90 of the switching controller
U.sub.2.
[0030] Further, while the first clamper Z.sub.1 is short-circuit,
the internal control circuit 90 will operate normally if the
voltage across the holding capacitor C.sub.ST remains higher than
the second clamping voltage V.sub.2. On the contrary, if the
voltage across the holding capacitor C.sub.ST is lower than the
second clamping voltage V.sub.2, the second clamper Z.sub.2 will
not breaks down. This causes the transistor 40 to be turned on and
the first P-transistor 61, the switch 63, and the first
N-transistor 65 to be turned off. The internal control circuit 90
will not be powered and will operate till the voltage across the
holding capacitor C.sub.ST reaches the first clamping voltage
V.sub.1 again.
[0031] As described above, the first clamping voltage V.sub.1 and
the second clamping voltage V.sub.2 serve as the hysteresis
threshold voltage range for the switching controller U.sub.2. That
is, the internal control circuit 90 only starts to operate as the
voltage across the holding capacitor C.sub.ST is higher than the
first clamping voltage V.sub.1, and stops the operation as the
voltage across the holding capacitor C.sub.ST is lower than the
second clamping voltage V.sub.2.
[0032] The buffer circuit 82 is controlled by the first
N-transistor 65 for turning on the switch 63 and turning off the
transistor 40. As the switch 63 is turned on, the internal control
circuit 90 is powered by the holding capacitor C.sub.ST. Meanwhile,
as the transistor 40 is turned off, the charging path from the
input voltage V.sub.IN to the holding capacitor C.sub.ST is cut off
for reducing power consumption of the power converter.
[0033] After the power converter starts up, the first N-transistor
65 will be turned off once the output of the power converter is
short-circuit. This could result in the auxiliary winding N.sub.A
of the transformer T.sub.1 failing to provide energy to the holding
capacitor C.sub.ST via the diode D.sub.A. Since the input of the
first inverter 71 is pulled to the ground reference, the first
inverter 71 will output a high-level voltage. After the transistor
40 is turned on, the power converter will operate again.
[0034] FIG. 4 shows a circuit diagram of the power converter having
the start-up apparatus according to a second embodiment of the
present invention. The start-up apparatus comprises a transistor
45, a buffer circuit 82a and a switch 63. The transistor 45 of the
second embodiment, which is a metal oxide semiconductor field
effect transistor (MOSFET), replaces the transistor 40 of the first
embodiment.
[0035] The buffer circuit 82a comprises the second P-transistor 62,
the first inverter 71, the third resistor R.sub.3, a second
inverter 72 and a second N-transistor 73. The connection of the
second P-transistor 62 and the third resistor R.sub.3 in the second
embodiment is the same as that in the first embodiment of the
invention. An output of the first inverter 71 is connected to an
input of the second inverter 72. An output of the second inverter
72 is connected to a gate of the second N-transistor 73. The second
N-transistor 73 further has a source connected to the ground
reference and a drain connected to a gate of the transistor 45.
[0036] Now please refer to FIG. 5, wherein an equivalent circuit
diagram of the transistor 45 having parasitic capacitors is
illustrated. There structurally exist parasitic capacitors between
terminals of a MOSFET. A parasitic capacitor 91 is connected
between a drain and the gate of the transistor 45. A parasitic
capacitor 92 is connected between the gate and a source of the
transistor 45. A parasitic capacitor 93 is connected between the
drain and the source of the transistor 45. When a voltage V.sub.D
is applied to the transistor 45, voltage drops will be formed among
the terminals of the transistor 45. A voltage drop V.sub.GS between
the gate and the source of the MOSFET is given by, V GS = [ C 91 C
91 + C 92 ] .times. V D ( 4 ) ##EQU2##
[0037] While the voltage V.sub.D is sufficient large, there will
result in a sufficient gate-to-source voltage V.sub.GS to drive the
transistor 45 on.
[0038] Owing to the characteristic of the transistor 45 described
above, the input voltage V.sub.IN can drive the transistor 45 on to
charge up the holding capacitor C.sub.ST. As the voltage across the
holding capacitor C.sub.ST reaches the first clamping voltage
V.sub.1, the first clamper Z.sub.1 and the second clamper Z.sub.2
will break down and a voltage V.sub.R will be generated across the
second resistor R.sub.2. The voltage V.sub.R turns on the first
N-transistor 65. This turns on the first P-transistor 61, the
second P-transistor 62, the switch 63, and the second N-transistor
73. Since the second N-transistor 73 is turned on, the gate of the
transistor 45 will be pulled to the ground reference, which turns
off the transistor 45. The charging path from the input voltage
V.sub.IN to the holding capacitor C.sub.ST is cut off accordingly
so as to save power consumption of the power converter.
[0039] Referring to FIG. 4, after the power converter starts up,
the first N-transistor 65 is turned off once there is a
short-circuit at the output of the power converter. Therefore, the
auxiliary winding N.sub.A of the transformer T.sub.1 fails to
provide energy to the holding capacitor C.sub.ST. Meanwhile the
first N-transistor 65 is turned off and the input of the first
inverter 71 is pulled to the ground reference, afterward, the
voltages at the output of the first inverter 71, the input of the
second inverter 72, and the control terminal of the switch 63
become high. This causes the switch 63 to be turned off and results
in a floating connection of the drain of the second N-transistor 73
and the gate of the transistor 45. While the input voltage V.sub.IN
is continuously applied to the power converter, the power converter
will restart to operate.
[0040] Referring to FIG. 4, a circuit diagram of the power
converter having the start-up apparatus according to a third
embodiment of the present invention is illustrated in FIG. 6. A
buffer circuit 82b comprises the second P-transistor 62, the first
inverter 71, the second inverter 72, the second N-transistor 73,
the third resistor R.sub.3 and a charge pump capacitor C.sub.1. The
charge pump capacitor C.sub.1 has a positive terminal connected to
the gate of the transistor 45 and the drain of the second
N-transistor 73. The charge pump capacitor C.sub.1 further has a
negative terminal connected to the output of the first inverter 71
and the input of the second inverter 72. A voltage divider formed
by resistors R.sub.A and R.sub.B further connects to the source of
the second N-transistor 73. The voltage divider is connected
between the first terminal of the switch 63 and the ground
reference. A junction of resistors R.sub.A and R.sub.B is further
connected to the source of the second N-transistor 73.
[0041] Further referring to FIG. 6, the input voltage V.sub.IN
drives the transistor 45 on to charge up the holding capacitor
C.sub.ST. This results in a low-level driving voltage V.sub.X at
the output of the first inverter 71. The second inverter 72 will
then turn on the second N-transistor 73. The voltage across the
holding capacitor C.sub.ST is coupled to the voltage divider in
parallel. Moreover, since the second N-transistor 73 is turned on,
voltages at the source of the second N-transistor 73, the drain of
the second N-transistor 73, and the gate of the transistor 45 will
be equal to a junction voltage at the junction of resistors R.sub.A
and R.sub.B. The junction voltage will start to charge up the
charge pump capacitor C.sub.1. In the meantime, the voltage at the
gate of the transistor 45 is lower than the voltage at the source
of the transistor 45. This turns off the transistor 45 and
therefore cuts off the charging path from the input voltage
V.sub.IN to the holding capacitor C.sub.ST for reducing power
consumption of the power converter.
[0042] In FIG. 6, once there is a short-circuit at the output of
the power converter, the auxiliary winding N.sub.A of the
transformer T.sub.1 is unable to charge the holding capacitor
C.sub.ST via the diode D.sub.A. This causes the first N-transistor
65 to be turned off, and makes the first inverter 71 output a
high-level driving voltage V.sub.X. The second N-transistor 73 will
be turned off. The driving voltage V.sub.X adds up the voltage
across the charge pump capacitor C.sub.1 to turn on the transistor
45. Accordingly, the power converter operates again.
[0043] Referring to FIG. 3, and a circuit diagram of the power
converter having the start-up apparatus according to a fourth
embodiment of the present invention is illustrated in FIG. 7. This
embodiment is implemented by removing the transistor 40 in the
first embodiment of the present invention. A first terminal of the
start-up resistor R.sub.ST is supplied with the input voltage
V.sub.IN and a second terminal of the start-up resistor R.sub.ST is
connected to the holding capacitor C.sub.ST to build a RC
charge/discharge circuit. The rest operation is the same as that in
the first embodiment of the present invention and will be described
as follows.
[0044] As the input voltage V.sub.IN is applied to the power
converter, the holding capacitor C.sub.ST will be charged up. When
the voltage across the holding capacitor C.sub.ST exceeds a
start-up voltage, the first clamper Z.sub.1 and the second clamper
Z.sub.2 will break down successively and a voltage V.sub.R will be
built across the second resistor R.sub.2. The voltage V.sub.R turns
on the first N-transistor 65, therefore, the gate of the first
P-transistor 61, the output of the buffer circuit 82, and the
control terminal of the switch 63 will be pulled to the ground
reference. This turns on the first P-transistor 61 and the switch
63. Since the first P-transistor 61 is turned on, the first clamper
Z.sub.1 is short circuit. While the switch 63 is turned on, energy
provided from the auxiliary winding N.sub.A of the transformer
T.sub.1 to the holding capacitor C.sub.ST powers the internal
control circuit 90.
[0045] Further referring to FIG. 7, the buffer circuit 82 comprises
a second P-transistor 62 having a gate connected to the gate of the
first P-transistor 61 and the source connected to the holding
capacitor C.sub.ST; a first inverter 71 having an input connected
to a drain of the second P-transistor 62 and an output connected to
a control terminal of the switch 63; and a third resistor R.sub.3
having a first terminal connected to the drain of the second
P-transistor 62 and a second terminal connected to the ground
reference.
[0046] While the first clamper Z.sub.1 is short-circuit, the
internal control circuit 90 operates normally if the voltage across
the holding capacitor C.sub.ST is higher than the second clamping
voltage V.sub.2. On the contrary, if the voltage across the holding
capacitor C.sub.ST is lower than the second clamping voltage
V.sub.2, the second clamper Z.sub.2 will not break down anymore.
This turns off the first P-transistor 61, the switch 63, and the
first N-transistor 65. Accordingly, the internal control circuit 90
will stop operation. The power converter will restart to operate as
the voltage across the holding capacitor C.sub.ST exceeds the
start-up voltage.
[0047] As described above, the first clamping voltage V.sub.1 and
the second clamping voltage V.sub.2 provide a hysteresis threshold
voltage range. That is, the internal control circuit 90 only
operates as the voltage across the holding capacitor C.sub.ST is
higher than the first clamping voltage V.sub.1 and only stops
operation when the voltage across the holding capacitor C.sub.ST is
lower than the second clamping voltage V.sub.2.
[0048] The buffer circuit 82 operates in response to the switching
of the first N-transistor 65. While the buffer circuit 82 turns on
the switch 63, the energy in the holding capacitor C.sub.ST
provided from the auxiliary winding N.sub.A of the transformer
T.sub.1 is able to power the internal control circuit 90.
[0049] In short, after the power converter starts to operate, the
start-up apparatus cuts off the charging path from the input
voltage V.sub.IN to the holding capacitor C.sub.ST for saving power
consumption of the power converter. Meanwhile, the power
consumption under the condition without power loading can be
improved to meet the green-mode specification. The start-up
apparatus also provides a hysteresis threshold voltage range for
turning on/off the power converter.
[0050] It will be apparent to those skilled in the art that various
modifications and variations can be made to the structure of the
present invention without departing from the scope or spirit of the
invention. In view of the foregoing, it is intended that the
present invention covers modifications and variations of this
invention provided they fall within the scope of the following
claims and their equivalents.
* * * * *